Compound semiconductor devices have been developed in which a GaN layer and an AlGaN layer are formed over a substrate and the GaN layer is used as an electron transport layer. One such compound semiconductor device includes GaN-based high electron mobility transistors (HEMTs).
GaN has superior electrical characteristics. For instance, since GaN has a high saturation electron velocity and a wide band gap, it has a high breakdown voltage characteristic. GaN also has a wurtzite crystalline structure and a polarity in a <0001> direction parallel to the c-axis. Furthermore, with the heterostructure of the GaN layer and the AlGaN layer, piezoelectric polarization is induced in the AlGaN layer due to the lattice distortion of both layers, generating two-dimensional electron gas (2 DEG) of high concentration at an interface between the GaN layer and the AlGaN. Due to the above-described reasons, GaN has been given attention as a material of high-frequency devices and a device for electric power.
However, it is very difficult to manufacture a GaN substrate with a good crystallinity. Major conventional solutions have been such as forming a GaN layer, AlGaN layer and so forth by hetero-epitaxial growth, over a Si substrate, sapphire substrate, SiC substrate or the like. In particular as for Si substrate, those having large diameter and high quality are readily available at low costs. Investigations into structures, having a GaN layer and an AlGaN layer formed over the Si substrate, have therefore been flourishing.
However, it is difficult to suppress a leakage current in a GaN-based HEMT with a Si substrate. Though an investigation has been conducted in which Fe is doped into a lower portion of an electron transport layer in order to a leakage current, doped Fe decreases crystallinity of the electron transport layer and affects other characteristics.
[Patent Literature 1] Japanese Laid-Open Patent Publication No. 2011-23642
[Patent Literature 2] Japanese Laid-Open Patent Publication No. 2010-225710
[Patent Literature 3] Japanese Laid-Open Patent Publication No. 2011-187654
[Patent Literature 4] Japanese Laid-Open Patent Publication No. 2011-228442